Ca regulates carbohydrate metabolism and energy production
As shown in Table 1, under different Ca levels, the expression of a large scale of carbohydrate metabolism and energy production related proteins were influenced. PPI network also enriched a carbohydrate metabolism function module (Figure 5). The majority of these proteins showed increased abundance under high Ca treatment compared with low Ca or medium Ca treatment. The involved metabolism pathways of these proteins included tricarboxylic acid cycle (TCA), Embden-Meyerhof-Parnas pathway (EMP) and pentose phosphate pathway (PPP).
The critical roles of carbohydrate metabolism to plant growth and development have been well reviewed by previous reports as carbohydrate metabolism provide material and energy premise for plant growth and development.37,38High Ca level may benefit carbohydrate metabolism related processes inP. massoniana seedlings through up-regulating the expression of relevant protein, and resulted in flourish phenotype and increased biomass (Figure 1). The surging expression of two PPP related proteins, D-ribulose-5-phosphate-3-epimerase isoform 3 (spot 4) and fructose-bisphosphate aldolase (spot 39) under high Ca treatment manifests elevated PPP process play a role in exogenous Ca supportedP. massoniana seedling growth. PPP is a major source for reduced nicotinamide adenine dinucleotide phosphate (NADPH) and substrates for biosynthetic processes.39 Exogenous Ca probably accelerated the production of NADPH and metabolic intermediates, which laid a foundation for various biosynthesis processes. The detected up-regulated multiple assimilation and biosynthesis related proteins, such as nitrogen assimilation and amino acid synthesis, secondary metabolites synthesis related proteins (Table 1) verified this speculation to some extent.
Energy metabolism and ATP production are basic biological activities in living organism.40 The increased expression of a number of ATP synthase subunits (spots 29, 31, 32, 34 ) revealed high Ca treatment promoted the accumulation of ATP and the storing of energy in P. massoniana seedlings. In the PPI analysis, we noticed ATP synthase subunit beta have interactions with all the four obtained function modules (Figure 5). It is broadly accepted that both photosynthesis and some carbohydrate metabolism processes such as TCA are ATP metabolism, especially ATP biosynthesis related processes. Chaperones participate in a number of biological processes in plant, including oxidative phosphorylation process for ATP production.41 In the oxidative phosphorylation process, the major approach producing ATP, reactive oxygen species (ROS) are unavoidable by-products and can be regulated by anti-oxidation system.42 High Ca promoted ATP biosynthesis in P. massoniana is inferred to be a synergistic result of carbohydrate metabolism and photosynthesis, and probably have crosstalk with other biological processes such as ROS metabolism.